Broad band antenna

ABSTRACT

An antenna (1) comprises a first elongate antenna element (3) for connection, in use, to a signal carrying element of an apparatus with which the antenna is used, and a shorter second antenna element (5) extending substantially around a proximal end portion (9) of the first antenna element (3), a proximal end (6) of the second element (5) being connected to the proximal end (8) of the first element. A body (12) of a dielectric material is disposed between the first and second antenna elements (3,5). The antenna (1) has at least two optimum operating frequencies, each with an associated usable bandwidth, which bandwidths preferably overlap. The antenna (1) may include further antenna elements (20,22) which further extend the overall usable bandwidth (B u ) of the antenna (1). One or more &#34;ground plane&#34; elements (16,17) are preferably also provided in the antenna for decoupling the first and second antenna elements (3,5) from an earthed screen or element in the apparatus with which the antenna is used.

BACKGROUND OF THE INVENTION

This invention relates to antennae which are capable of being used forsignal transmission and/or reception purposes at radio and microwavefrequencies.

Antennae capable of operating in the radio and/or microwave frequencyrange are commonly attached to, or incorporated within, electricalequipment for use in the home, commercial or laboratory environments,such as televisions, radios, cellular telephones, and wireless localarea networks (WLANS). One known type of antenna is the conventional"loop-type" antenna which essentially consists of a tuned loop of wiredesigned to receive or radiate signals at frequencies falling within arelatively narrow usable frequency bandwidth which may, typically, beapproximately 50-60 MHz wide. Such antennae have the disadvantage ofhaving directional radiation properties the effect of which is that theantenna needs careful orientational adjustment in order to operateeffectively. Moreover, the antenna is incapable of receiving orradiating signals at frequencies falling outside its relatively narrowoperational bandwidth.

Another known type of antenna is the "quarter wave" antenna whichgenerally consists of an elongate antenna element whose length is chosento be equal to one quarter of the wavelength at the desired optimumoperating frequency of the antenna and which operates against anelectrical ground which is usually provided by the earthed outerconducting element of a coaxial cable, the antenna element beingconnected to the central signal carrying element of the cable. Althoughsuch antennae have omni-directional radiation characteristics, they alsohave the disadvantage of operating effectively only over a fixed,relatively narrow frequency bandwidth. The antenna element of suchquarter-wave antennae may often be telescopic such that the length ofthe element may be varied by a user. Such antennae still require carefuladjustment by the user in order to operate at different frequencieswhich can be time consuming and frustrating for the user. Such variablelength antennae are, moreover, unsuitable for use in, for example, localarea networks and cellular radio communications applications whereconstant antenna adjustment is not a viable option for maintainingeffective operation.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome one or more of theforegoing disadvantages and to provide an antenna having a relativelybroad usable bandwidth and substantially omni-directional radiationcharacteristics.

According to the present invention, an antenna comprises a firstelongate antenna element of fixed predetermined length, a proximal endof which is electrically connected, in use, to a signal carryingconducting element of an apparatus with which the antenna is used, and asecond antenna element of fixed predetermined length, a proximal end ofwhich is electrically connected to the proximal end of the first antennaelement, the length of the second antenna element being substantiallyless than the length of the first antenna element and the second antennaelement being formed and arranged so as to extend substantially around aproximal end portion of said first antenna element so as tosubstantially screen said proximal end portion, with the proximal endportion of the first antenna being supported by a body of dielectricmaterial disposed between said first and second antenna elements, saidproximal end portion of the first antenna element being electricallyinsulated from said second antenna element by said body of dielectricmaterial.

One advantage of the antenna of the present invention is that it has twooptimum operating frequencies (i.e. resonant frequencies), each with anassociated usable bandwidth. This is due to the fact that in combinationsaid first and second antenna elements operate together like a singlequarter wave antenna having a total length equal to the distance from adistal end of the first element to the signal carrying conductingelement of the apparatus with which the antenna is used, which lengthdetermines the resonant frequency of the combined antenna, and thesecond antenna element also operates as a quarter wave antenna whoselength determines its own, different, resonant frequency. Moreover, likequarter wave antennae, the antenna of the present invention hassubstantially omni-directional radiation characteristics.

The term "usable bandwidth" is used herein and throughout this documentto mean the frequency bandwidth over which the antenna return loss issufficiently low to produce acceptable antenna performance where theantenna is used with commercially produced equipment. Conveniently thefirst antenna element simply consists of a length of low-loss conductingwire. Preferably the second antenna element is of generally tubularform. The usable bandwidth associated with the second antenna element onits own is greater that that associated with the first and secondantenna elements combined, due to the relatively lower inherent Q-factorof the second antenna element on its own resulting from its generallytubular form.

Preferably, the length of the first and second antenna elements are suchthat the ranges of operating frequencies within the aforesaid usablebandwidths associated with the two elements overlap. In this manner anantenna having a relatively broad overall usable bandwidth is provided.In general, the ratio of the lengths of the first and second antennaelements may be from 4:1 to 4:3, for example about 3:2.

Advantageously, the body of the dielectric material which supports theportion of the first antenna element located within the generallycylindrical body of the second antenna element protrudes beyond a distalend of the second antenna element, part of the way towards a distal endof the first antenna element. The protruding length of dielectricmaterial provides some impedance matching of electromagnetic (e.m.)signal waves along the length of the antenna, between a distal portionof the first antenna element (which has a relatively low e.m. waveimpedance) and the distal end of the second antenna element (which has arelatively high e.m. wave impedance). This helps to produce improvedantenna return-loss performance towards the lower frequency end of theusable bandwidth of the antenna. The dielectric material may be apolyethylene-type plastics material.

Advantageously, the antenna further comprises at least one "groundplane" element which is connected at a proximal end, in use of theantenna, to an electrically earthed (in radio frequency signal terms)screen or element provided in the apparatus with which the antenna isused. Preferably the ground plane element(s) extend(s) generallyperpendicularly to the first and second antenna elements. The groundplane element(s) is/are formed and arranged so as to substantiallydecouple the first and second antenna elements from the earthed screenor element in the apparatus with which the antenna is used, at at leastone or more operating frequency within the usable bandwidth of theantenna. In this way the ground plane element prevents radio frequency(RF) currents being induced in the aforementioned earthed screen elementby the radiation field of the antenna (such currents could produceundesirable electrical interference or feedback effects in theelectrical equipment with which the antenna is used).

This may conveniently be achieved by providing at least two ground planeelements each in the form of a generally elongate element extendinggenerally perpendicularly to the first and second antenna elements andat substantially one hundred and eighty degrees to each other and eachconnected, in use, at a respective proximal end to the electricallyearthed screen or element of the apparatus within which the antenna isused. Preferably, the electrical length from a distal end of each groundplane element to the proximal end thereof is substantially equal to onequarter of the wavelength at a frequency which falls in a middle regionof the usable bandwidth of the antenna. The two ground plane elementshelp provide a generally symmetrical spatial distribution of capacitancebetween the first and second antenna elements and electrical ground.Alternatively, a single ground plane element is provided in the form ofsolid disc of conducting material electrically connected at its centreto the electrically earthed screen or element of the apparatus withwhich the antenna is used, the diameter of the disc being substantiallyequal to one half of the wavelength at a frequency which falls in amiddle region of the usable bandwidth of the antenna. Other forms ofground plane are also possible and may comprise, for example one or morehelical conducting elements.

An outer surface of the generally cylindrical body of the second antennaelement is preferably provided with a sleeve of insulating material. Theantenna preferably also includes one or more further antenna elementswhich further extend the usable bandwidth of the antenna and/or improveantenna performance at its upper frequency range. Each of the furtherantenna elements may be in the form of a generally elongate conductingelement, which is preferably a low-loss wire, which is electricallyconnected at a proximal end thereof to the proximal end of the firstantenna element. Advantageously, each further element extends betweenthe first element and the second element, from the latter's proximal endto its distal end. The presence of the extra wires within the hollowbody of the second antenna element acts to produce increased efficiencyof antenna operation in the usable bandwidth associated with the secondantenna element. The further elements may each pass between the body ofdielectric material and the inner surface of the second antenna element,or, preferably, they pass through the body of dielectric materialitself, until they emerge at the distal end o the second antennaelement. In a preferred embodiment, the body of dielectric materialprotrudes from the distal end of the second antenna element and thefurther antenna elements each pass through the generally tubular body ofthe second antenna element in the dielectric and emerge at a distal endof the protruding portion of dielectric. At the point where they emergefrom the second antenna element, or the dielectric, the further elementsare preferably returned towards the proximal end of the first and secondantenna elements so as to lie substantially parallel to the outersurface of the second antenna element with electrically insulatingmaterial therebetween. The length of each of the further antennaelements is chosen to be such that the free end of each reaches back atleast part of the way along the length of the second antenna element.

Where a plurality of further elements are provided they areadvantageously each of a different predetermined length. The furtherelement(s) each operate to provide a respective resonant frequency ofthe antenna, with a respective associated usable bandwidth. This is dueto an open-circuit transmission line effect produced between eachfurther antenna element and the second antenna element. The length ofeach further element from its free end to the point where it is levelwith the distal end of second antenna element determines the respectiveresonant frequency of the antenna. Certain lengths will cause theantenna to resonate at frequencies above the optimum operating frequencyof the second antenna element. Some lengths may provide resonantfrequencies below this frequency.

Preferably a plurality of such further antenna elements is provided andeach of the elements is electrically connected to the first antennaelement where they emerge from the distal end of the protruding portionof the dielectric material. This ensures that all the further elementsand the first antenna element are held at the same e.m. wave RFpotential at this connection which, in turn, ensures that the relativeopen-circuit transmission line effects produced by the further elementsare not dependent upon the antenna's surrounding environment and/orspurious RF signals to which the further elements may be subjected.

In an alternative embodiment, the further antenna elements, which mayconveniently each consist of a predetermined length of low-loss wire,have one end which is electrically connected to the first antennaelement where it emerges from the dielectric material protruding fromthe second antenna element, the further element being positionedrelatively parallel to the second antenna element with the free end ofthe further element reaching partly down it. This arrangement producessimilar open-ended transmission line effects between each furtherelement and the main body of the antenna comprising the first and secondantenna elements.

The antenna of the present invention can thus provide an exceptionallybroad usable bandwidth, for example, 200-1200 MHz, due to the combinedplurality of resonant frequencies and associated bandwidths provided bythe various antenna elements. The antenna has also been found to havegood return-loss characteristics within its usable bandwidth,particularly where said further antenna elements are employed.

The second antenna element, protruding body of dielectric material,remaining unsupported portion of the first antenna element, and furtherantenna elements (if provided), may all be encased together in aninsulating sleeve. The sleeve provides support and protection to theantenna elements. The ground plane elements are, preferably, also eachencased in an insulating sleeve. A coaxial feed cable may be providedfor use with the antenna for connecting the antenna to the apparatuswith which it is to be used. Where such a feed cable is provided, thefirst antenna element is connected at its proximal end to a centralsignal carrying element of the feed cable and the ground planeelement(s) are connected at their proximal end(s) to an outer screenelement of the feed cable which element will, in RF signal terms, beelectrically earthed, in use.

The relative lengths of the various antenna elements may be scaled up ordown in order to provide an antenna for operating in a particularfrequency range. Where lower frequencies are to be used (e.g.approximately 170 MHz, as used in communications application) and thuslonger antenna element lengths are necessary, other forms of antennaelement may be employed, such as helical elements, in order to retain arelatively compact overall antenna structure. For example, the generallycylindrical body of the second antenna element may comprise a coil oflow-loss wire surrounding a portion of the first antenna element. Thefirst antenna element may also include a helical portion, for example,towards the distal end of the element. Thus either or both of the firstand second antenna elements could be partly or wholly helical.

Similarly, the antenna design could be scaled down in size for operationin higher frequency ranges e.g. up to 1.5 GHz for use in the microwaverange, the upper frequency limit being restricted only by the geometricpracticalities of implementation.

The antenna is thus suitable for use in many RF and microwave frequencyapplications including high-speed digital computer-to-computer datahighways using RF-link technology, automatic interrogation of vehicleson motorways and, in particular, at toll booths, as well as morewidespread application in television, radio reception and radiocommunication including cellular radio. References to RF signalshereinbefore should therefore be taken to include a reference tomicrowave signals. Preferred embodiments of the invention will now bedescribed by way of example only and with reference to the accompanyingdrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an antenna according to oneembodiment of the present invention;

FIG. 2a is a logarithmic graph of signal return loss against frequency,for the antenna of FIG. 1;

FIG. 2b is a graph of voltage standing wave ratio against frequency, forthe antenna of FIG. 1;

FIG. 3 is a schematic illustration of an antenna according to an otherembodiment of the invention;

FIG. 4a is a schematric illustration of an open-circuited transmissionline formed by elements of the antenna of FIG. 3;

FIG. 4b is a circuit diagram of a series resonant circuit representingthe open-circuited transmission line of FIG. 4a;

FIG. 5a is a logarithmic graph of signal return loss against frequency,for the antenna of FIG. 3; and

FIG. 5b is a graph of voltage standing wave ratio against frequency, forthe antenna of FIG. 3.

DESCRIPTION OF ILLUSTRATED EMBODIMENT

An antenna 1, shown schematically in FIG. 1 of the drawings, comprises afirst elongate antenna element 3 which consists of a low losselectrically conductive wire, such as copper, and a second antennaelement 5 which consists of a hollow generally cylindrical body oflength L1 and made of copper braiding. At a proximal end 6 of the secondantenna element 5 the copper braiding is "pig-tailed" to form arelatively thin, twisted end 7 which is electrically connected, forexample by soldering, to a proximal end 8 of the first element 3. Thegenerally cylindrical body of the second element 5 surrounds a proximalportion 9 of the first antenna element 3 and the length of the firstantenna element from the proximal end 6 of the second antenna element toits distal end 10 is L3 which is substantially greater than L1.

A generally cylindrical body 12 of dielectric material is disposedbetween the first and second antenna elements 3, 5 and surrounds aportion of the first antenna element which includes the proximal portion9 disposed within the generally cylindrical body of the second antennaelement 5. The dielectric material used is a polyethylene plasticsmaterial commonly used as the dielectric material in conventionaltelevision (TV) coaxial cable. Other types of dielectric material could,however, be used. The length of the dielectric body 11 is longer thanthe length L1 of the generally cylindrical body of the second antennaelement and is disposed within the second antenna element such that aportion of length L2 of the dielectric protrudes from a distal end 11 ofthe second antenna surrounding a corresponding length L2 of the firstantenna element 3. The proximal end 8 of the first antenna element iselectrically connected, in use of the antenna, to a conducting element15 for carrying an electrical signal, which element may, for example, bethe central wire of a coaxial feed cable 14 (as shown in FIG. 1),connected to, or incorporated in, the electrical apparatus (e.g.television, radio etc) with which the antenna 1 is used.

The antenna further includes two ground plane elements 16, 17 respectiveproximal ends of which are connected, in use to an electrically earthed(in RF signal terms) screen or element, which may as shown in FIG. 1 bethe outer earth screen element 13 of a coaxial cable connected to, orincorporated within, the apparatus with which the antenna 1 is used.

The antenna operates, in use, according to the principle of a "quarterwave" antenna, the theory of which is that a signal carrying conductingelement of length equal to one quarter of the wavelength of apredetermined (desired) optimum operating frequency of the antenna,operating against an associated earth or "ground plane", will behavelike a conventional half-wave dipole antenna (due to the so called"mirror image" effects produced by the ground plane) and will resonateat the predetermined optimum operating frequency, this being theresonant frequency of the antenna.

In the antenna of FIG. 1 the complete antenna 1 has a first resonantfrequency which is determined by the length of the complete antenna fromthe distal end 10 of the first antenna element 3 to the proximal end ofthe antenna where the first antenna element is connected to theconducting element 15. This first resonant frequency is equal to thesignal frequency at which the total length of the complete antenna isapproximately equal to one quarter of the signal wavelength; the actualresonant frequency may vary slightly from the theoretical value due tocapacitance effects between antenna and ground and due to the fact thatthe antenna is not a single, perfect, continuous quarter wave antennaelement. The first resonant frequency of the antenna has an associatedusable bandwidth. The antenna 1 also has a second, higher, resonantfrequency determined by the length L1 of the generally cylindrical bodyof the second antenna element 5 which is deliberately chosen to besubstantially less than the length of the first antenna element, asshown in FIG. 1. The length from the distal end 11 of the second antennaelement to the connection between the first and the conducting element15 is similarly approximately equal to one quarter of the wavelength ofthe resonant frequency associated with the second antenna element. Theusable bandwidth with this second resonant frequency of the antenna 1 isslightly broader than that associated with the first resonant frequencyas the second antenna element behaves like a resonant circuit having aninherently lower Q-factor than that associated with the complete antennaincluding the first and second elements 3,5. This is primarily due tothe second antenna element's predominantly hollow, cylindrical form.

In use, the generally cylindrical body of the second antenna element 5substantially decouples the proximal portion 9 of the first antennaelement 3 within the cylindrical body. The open end presented by thedistal end 11 of the second element 5 presents a relatively high (e.m.wave) impedance to signals in the antenna, while the exposed, freeportion of the first element 3 located beyond the protruding portion ofthe dielectric body 12 presents a relatively low impedance. Theprotruding portion of length L2 of the dielectric acts to provide someimpedance matching which improves the overall performance of the antennain the lower frequency range usable bandwidth associated with the fulllength of the antenna.

The relative lengths of the two antenna elements 3, 5 are chosen suchthat the two usable bandwidths of the antenna partially overlap thusproviding a relatively broad overall usable bandwidth. This isillustrated by FIGS. 2a and 2b. FIG. 2a is a logarithmically presentedgraph of antenna signal return loss (a measure of power loss due tocurrent reflections occurring in the antenna), in decibels (dB), againstsignal frequency, F.

The usable bandwidth of the antenna extends largely over the region inwhich the plotted line P deviates substantially from the straightreference line X. FIG. 2b is a graph of the voltage standing wave ratio,VSWR (i.e. Maximum to minimum RF signal voltage measurable in theantenna) against signal frequency, F. The illustrated usable bandwidthB_(u) of the antenna extends from approximately 250 MHz to 900 MHz.These graphs were obtained with antenna element lengths L1, L2, L3approximately equal to 92, 30 and 255 millimeters respectively, and withground plane elements 16, 17 each of length approximately 110millimeters. The diameters of the first and second antenna elements 3, 5are approximately 1.5 millimeters and 5 millimeters respectively and thediameter of the dielectric body 12 is approximately 4.2 millimeters.

The ground plane elements 16, 17 operate to decouple the first andsecond antenna elements 3, 5 from the earthed screen or element of theapparatus and/or feed cable to which the antenna is connected, thuspreventing unwanted RF currents being induced therein (which couldinterfere with the operation of the apparatus itself). This may besubstantially achieved by choosing the length of the ground planeelements such that they will resonate (in the same way as a quarter waveantenna element) when currents are induced therein at a frequencyfalling towards the middle region of the usable bandwidth B_(u) of theantenna. In the antenna analyzed in FIGS. 2a and 2b, the lengths of theground plane elements were 110 millimeters each. Alternative forms ofground plane are possible, such as helical elements or a flat metallicdisc, the first and second antenna elements extending perpendicularlyaway from the centre of the helical elements or disc. Any number ofground plate elements may be used. By providing a symmetrical groundplane arrangement capacitances between the first and second antennaelements and ground are spatially symmetrically distributed, givingenhanced antenna performance.

To design an antenna 1 which will operate over a different frequencyrange i.e. the usable bandwidth is in a higher or lower frequency range,the relative lengths and sizes of the various antenna elements maysimply be scaled up or down accordingly. Other forms of antenna elementmay also be used, for example, in lower frequency ranges where therequired lengths of the first and second antenna elements are greater,helical elements may be used to retain a relatively compact antennaform. The second antenna element 5 could, for example, be a low-losscopper wire which is coiled into a cylindrical form surrounding the bodyof dielectric 12. In practice, it has been found that a piece of coaxialtelevision cable may be adapted to provide the first and second antennaelements and thinner diameter pieces of coaxial cable can be used as theground plane elements 16, 17.

FIG. 3 shows schematically an improved embodiment of the invention. Likeparts to those described with reference to FIG. 1 have been givenidentical reference numerals. The outer surface of the generalcylindrical body of the second antenna element 5 is covered in aninsulating sleeve (not shown). With the antenna element dimensions asgiven for the antenna analysed in FIGS. 2a and 2b, the diameter of theinsulating sleeve is approximately 6.5 millimeters. In this embodimentthe usable bandwidth has been further extended, and the performance ofthe antenna improved (i.e. in terms of efficiency), at the upperoperating frequency range of antenna, by the addition of two furtherantenna elements 20, 22. The further elements each consist of a lengthof low-loss (e.g. copper) wire, a proximal end of which is electricallyconnected to the proximal end 8 of the first antenna element 3. The bodyof dielectric 12 contains cellular air spaces or channels 21 throughrespective ones of which each of the further elements (and the firstelement) are threaded from a distal end 23 from which they emerge intothe surrounding air. The generally cylindrical body of the secondantenna element 5 surrounds a portion of the body of dielectric 12, asin the embodiment of FIG. 1. Where the two further elements 20, 22emerge from the dielectric 12 they are electrically shorted (e.g. bysoldering) to the first antenna element 3 where it emerges from thedielectric (at a position Y along the length of the first antennaelement). The remaining free distal portion of each of the furtherelements 20, 22 is bent back down the length L2 of the protrudingportion of the dielectric and over the insulated sleeve of the secondantenna element 5. The two further elements 20, 22 are of differentlengths and each terminates part of the way down the length L1 of thesecond antenna element, the lengths of the elements between the point Y,and their respective free end being L4 and L5 respectively. At the pointY where the further elements are joined to the first element, all threeelements are at the same signal RF potential (so relative effects due toenvironmental surroundings will not effect the relative RF potentials ofthe two further elements at the point Y). Viewed from this point andlooking down the antenna towards the proximal ends 6, 9 of the first andsecond antenna elements, each of the lengths L4 and L5 i.e. the bentback portions or "stubs" of the further elements 20, 22, and theparallel portion of the main body of the antenna, present anopen-circuited (open-ended) transmission line having a length l as shownin FIG. 4a. The length l is equal to the length of the respective stubbetween the distal end 11 of the second antenna element and the free endof the stub. For further elements 20, 22 the length l is equal to L4'for the longer element 20 and L5' for the shorter element 22, as shownin FIG. 3. The portion 30 of each transmission line disposed beyond thedistal end 11 of the second antenna element 5, adjacent to the portionof the first antenna element 3, presents a relatively high impedance(largely from the inductance of that portion of the transmission line)and the portion 32 of the line disposed between the open-end of thetransmission line and the distal end 11 of the second antenna elementpresents a relatively low impedance (largely due only to the capacitancebetween the stubs and the second antenna element 5). Each open-circuitedtransmission line may be represented by a series resonant circuitincluding a capacitor C and an inductor L, as shown in FIG. 4b. Thetotal impedance of each of the two transmission lines is different dueto the different stubs lengths L4, L5. The length L4', L5' of each stubbetween the distal end 11 of the second antenna element and the free endof that stub determines the capacitance between the stub and the secondantenna element 5. The longer the length of the stub, the greater thecapacitance.

The series resonant circuit represented by each open-circuitedtransmission line will resonate at a particular signal frequency whenthe reactances of the total inductance and capacitances of each circuitcancel out. This will be at a different frequency for each of the twotransmission lines due to the different lengths L4', L5' of the stubs.In the antenna of FIG. 3 the resonant frequencies associated with thefurther elements 20, 22 will occur towards the upper frequency range ofthe usable bandwidth B_(u) and so the further elements operate tofurther extend the usable bandwidth of the antenna 1. The shorter lengthelement 22 has a higher associated resonant frequency than the longerlength element 20. This is illustrated in FIGS. 5a and 5b which arelogarithmically presented graphs of signal return loss againstfrequency, F, and signal voltage standing wave ratio, VSWR, againstfrequency, F, respectively. When compared with the graphs of FIGS. 2aand 2b (which are the same scale) it can be seen that the usablebandwidth B_(u) of FIG. 5b is much greater than that of FIG. 2b. Thestub lengths of the other elements 20, 22 were respectively L4=105millimeters and L5=84 millimeters (the other antenna dimensions were thesame in each analysis).

The usable bandwidth B_(u) of the antenna extends from approximately 250MHz to 1200 MHz in FIG. 5b, with relatively low signal volt age standingwave ratios (VSWR's) (unity being the ideal value) being achieved over alarge proportion of this bandwidth. Similarly, the graph in FIG. 5aindicates improved signal return loss values as compared with the graphof FIG. 2a, the deeper troughs indicating increased antenna efficiency(i.e. less signal power loss due to current reflections).

Any number of further elements may be employed to broaden the usablebandwidth of the antenna, a practical limit being reached only when thestub lengths required to further extend the bandwidth become very short.(The shorter the stub length, the higher the associated resonantfrequency of the antenna).

A relatively small-value fixed capacitance (not shown) may be connectedbetween the proximal end of the first antenna element 3 and the groundplane elements 16, 17 to further optimise the antenna's signal returnloss performance over the usable bandwidth of the antenna.

The first, second and further antenna elements may be encased in aninsulating sleeve S (shown in broken line in FIG. 1) to provide supportand protection to the antenna. The ground plane elements 16, 17 may besimilarly encased, all the elements being held together by joining allthe insulating sleeves together at their proximal ends to form asymmetrical, three forked antenna structure. The sleeve S may beconveniently made of PVC.

I claim:
 1. A broad band antenna (1) comprising a first elongate antennaelement (3) of a fixed predetermined length, a proximal end (8) of whichis electrically connected, in use, to a signal carrying conductingelement (15) of an apparatus with which the antenna (1) is used, and asecond antenna element (5) of fixed predetermined length (L1), aproximal end (6) of which is connected to the proximal end (8) of thefirst antenna element (3) via a direct conductive pathway therebetween,said proximal end (6) of the second antenna element (5) thereby beingelectrically connected to said signal carrying conducting element (15),the length (L1) of the second antenna element being substantially lessthan the length of the first antenna element (3) and the second antennaelement (5) being formed and arranged so as to extend substantiallyaround a proximal end portion (9) of said first antenna element (3) soas to substantially screen said proximal end portion (9), with theproximal end portion of the first antenna element (3) being supported bya body (12) of dielectric material disposed between said first andsecond antenna elements (3,5), said proximal end portion (9) of thefirst antenna element being electrically insulated from said secondantenna element (5) by said body (12) of dielectric material, whereinthe antenna has two optimum operating frequencies which are dependentupon the lengths of said first and second antenna elements, each saidoptimum operating frequency having an associated usable bandwidth, andwherein said predetermined lengths of said first and second antennaelements are such that said associated usable bandwidths together form acontinuous usable bandwidth (B_(u)) of the antenna.
 2. An antennaaccording to claim 1 wherein the first elongate antenna element (3)comprises a length of conducting wire.
 3. An antenna according to claim1 wherein the second antenna element (5) is of generally tubular form.4. An antenna according to claim 1, wherein said predetermined lengthsof the first and second antenna elements (3,5) are such that there areoverlapping ranges of operating frequencies within said usablebandwidths associated with said two optimum operating frequencies of theantenna.
 5. An antenna according to claim 1 wherein the ratio of thepredetermined lengths of said first and second antenna elements (3,5) isfrom 4:1 to 4:3.
 6. An antenna according to claim 5 wherein said ratiois approximately 3:2.
 7. An antenna according to claim 1 wherein thebody (12) of dielectric material protrudes beyond a distal end (11) ofthe second antenna element (5).
 8. An antenna according to claim 1wherein the dielectric material of said body (12) is polyethylene.
 9. Anantenna according to claim 1, further comprising at least one groundplane element (16) formed and arranged for connection, in use theantenna (1), to an electrically earthed element (13) of the apparatuswith which the antenna is used.
 10. An antenna according to claim 9wherein said one or more ground plane elements (16) are elongate andextend substantially orthogonally to the first and second antennaelements (3,5).
 11. An antenna according to claim 9 further comprisingtwo elongate ground plane elements (16,17) extending substantiallyorthogonally to the first and second antenna elements (3,5) and atsubstantially one hundred and eighty degrees to each other.
 12. Anantenna according to claim 11 wherein the electrical length of each ofthe said two elongate ground plane elements (16,17) is substantiallyequal to one quarter of the wavelength at a frequency which falls in amiddle region of said usable bandwidth (B.sub.μ) of the antenna (1). 13.An antenna according to claim 9 wherein a single ground plane element isprovided in the form of a generally planar element of electricallyconducting material extending in a radial manner from, and substantiallyperpendicularly to, said first elongate antenna element (3) and formedand arranged for connection substantially at its centre, in use of theantenna (1), to an electrically earthed element (13) of the apparatuswith which the antenna is used.
 14. An antenna according to claim 13wherein said single ground plane element is in the form of a disc havinga diameter substantially equal to one half of the wavelength at afrequency which falls in a middle region of said usable bandwidth(B.sub.μ) of the antenna.
 15. An antenna according to claim 9, whereinthe ground plane element is encased in a respective insulating sleeve.16. An antenna according to claim 1, further comprising one or moreadditional antenna elements (20,22) which extend said usable bandwidth(B_(u)) of the antenna (1).
 17. An antenna according to claim 16 whereinat least one of said additional antenna elements (20,22) is an elongateconducting element which is electrically connected at a proximal endthereof to the proximal end (8) of the first antenna element (3).
 18. Anantenna according to claim 17, wherein a plurality of such additionalelongate antenna elements (20,22) are provided and each additionalantenna element (20,22) is of a different predetermined length.
 19. Anantenna according to claim 16 wherein an outer surface of the generallycylindrical body of the second antenna element (5) is provided with asleeve of insulating material.
 20. An antenna according to claim 19,wherein at least one of said additional antenna elements passes throughthe body (12) of dielectric material, emerging at a distal end (23) of aportion thereof which portion protrudes beyond a distal end (11) of thesecond antenna element (5), from where said additional antenna element(20,22) returns towards the proximal ends (8,6) of the first and secondantenna elements (3,5) so as to lie substantially parallel to an outersurface of the second antenna element (5) with said sleeve of insulatingmaterial therebetween, a free end of said additional antenna elementreaching back at least a part of the way along the length (L1) of thesecond antenna element (5).
 21. An antenna according to claim 20 whereina plurality of such additional elongate antenna elements (20,22) areprovided and each of said additional antenna elements (20,22) iselectrically connected to the first antenna element (3) where saidadditional antenna element emerges from the distal end (23) of saidprotruding portion of the dielectric body (12).
 22. An antenna accordingto claim 1, wherein said first and second antenna elements (3,5) aretogether encased in an insulating sleeve (S).
 23. An antenna accordingto claim 22, wherein the insulating sleeve (S) is made ofpolyvinylchloride.
 24. An antenna according to claim 1, wherein thesecond antenna element (5) comprises a hollow, generally cylindricalbody made of copper braiding.